Narrowband interference, NBI, is an increasing problem for radio communication systems. In particular at low frequency bands since everyone wants to leverage on good propagation properties. Examples of NBI sources include e.g. TV transmitters, push-to-talk systems or unlicensed transmissions. NBI is also a problem in many other situations. One example is when re-farming Global System for Mobile communication, GSM, spectrum to Wideband Code Division Multiple Access, WCDMA. In this case the operator typically wants to keep some spectrum for GSM, while gradually increasing the bandwidth for WCDMA. Since many operators have limited spectrum this is of course challenging. As an example, assume that an operator has 6 MHz spectrum and wants to use 12 GSM carriers (2.4 MHz). Then there is 3.6 MHz left for WCDMA, which is significantly less than for a normal 5 MHz WCDMA carrier. Hence, even though WCDMA may use reduced bandwidth Rx and/or Tx filters, it is likely that GSM will create strong narrowband interference towards the WCDMA, and vice versa.
To mitigate narrowband interference, one approach is to filter out the interference by creating a narrow and deep Notch in the frequency part affected by the interference. This will hopefully remove the harmful interference without negatively affect the desired signal spectrum too much.
The Narrowband Interference Rejection, NBIR, may be implemented in different ways and the implementation may reside in radio, baseband or a combination thereof.
NBIR has been commercially available for WCDMA for a few years and has been a successful feature. Due to the benefits with NBIR, it is of interest to enhance the NBIR capability. For example, it would be desired to cover more diverse interference scenarios and remove potential problems with existing schemes. Examples of improvement areas include adding support for handling very strong interference, and to enhance the robustness/stability of the NBIR functionality.